can choose the end date before the start date when using the rang pincker

[vernon-lu]

when the first time to use DateRangePicker , the start date and the end date is empty,now you choose the start date, then you choose the end date, you will find you can choose the date before the start date, and vice versa.
I find the code in dateUpdated

if (new_date < this.dates[i]) {
        // Date being moved earlier/left
        while (i >= 0 && new_date < this.dates[i]) {
             this.pickers[i--].setUTCDate(new_date);
        }
}
else if (new_date > this.dates[i]) {
        // Date being moved later/right
        while (i < l && new_date > this.dates[i]) {
             this.pickers[i++].setUTCDate(new_date);
       }
}               

here must check whether the this.dates[i] is null

[acrobat]

Fixed in #1036